The past 25 years have seen a major paradigm shift in the field of violence prevention, from the assumption that violence is inevitable to the recognition that violence is preventable. Part of this shift has occurred in thinking about why violence occurs, and where intervention points might lie. In exploring the occurrence of violence, researchers have recognized the tendency for violent acts to cluster, to spread from place to place, and to mutate from one type to another. Furthermore, violent acts are often preceded or followed by other violent acts.